The quality of reproduced sound seldom satisfies those critics who have the ability to remember what real musical performances sound like. Over the years, the specifications of equipment in regard to frequency response and distortion have improved; yet there remain reputable critics who dislike the sound of the improved designs and point to the "musically correct" qualities of lessor specification designs, usually older vacuum tube equipment. This diversity of opinion has led to some investigations into design factors which improve the measured distortion but do not improve the quality of sound reproduction. The most common method for both reducing distortion and improving frequency response is to use some form of negative feedback.
For some time, most improvements in amplifier designs tended to involve the use of increasing amounts of negative feedback. Because of stability limitations, vacuum tube amplifiers seldom featured more than 20 db of feedback. With the advent of wideband transistors, designs using up to 60 db of feedback were perfected.
Otala showed that the stabilizing procedures used in designing amplifiers with large amounts of feedback led to increases in distortion for transient or fast slope signals and that this type of distortion which he called "Transient Intermodulation Distortion" or TIM was caused by loop negative feedback and did not occur if loop feedback was not used. As a result of this analysis, using less loop negative feedback became the rule, in some cases it was eliminated entirely. Alternative ways of improving frequency response and distortion without negative feedback have proven very expensive. When small amounts of negative loop feedback are used in a properly stabilized design, the amount of TIM should be minimal. Yet distortion can still be clearly heard. I have discovered that these residual distortions are not caused by the TIM fast slope mechanism but by non linear phenomina in supposedly passive circuit components. The elimination of these component produced distortions allows for designs using moderate amounts of loop feedback for optimum reduction of all types of distortion.
It is the object of the present invention to reduce the distortion of non linear passive circuit elements in amplifier circuits. A further object is to increase the quality of audio amplifiers at minimum cost, further objects will be evident by reading the specifications and drawings.